Rex Racer was a promising racer. He took the Mach 1 without Pops knowing. Pops was concerned when he found out. The car was still experimental and needed to be tested first. Some debris on the track caused Rex and the Mach 1 to crash. Rex made it out of the wreckage safely. When he blamed the crash on the car, Pops blew a gasket. Years of his work had been destroyed thanks to Rex. Pops decided that Rex was too immature to handle being a racer and threatened to take him off the racing team. That was fine with Rex. He decided he'd become a world champion on his own. He walked away from Pops and his family.
Rex later went to work for the International Secret Police. He was forced to go undercover and not even his family could know his true identity. As Racer X, he was both a professional driver while trying to take down the forces of evil.
In the original manga, at the end, Racer X revealed himself to his brother Speed, who always suspected Racer X could be his missing brother. His undercover mission was over and he could return to his family. He and Speed would become the world's best racing team.
In the anime, things ended differently. It was meant to be their final meeting, Speed asks Racer X point blank if he was his brother Rex. Rather than give him the answer he had been waiting for so long to hear, Racer X sucker punches Speed in the gut and knocks him out. He then decides to give up the guise of Racer X and leaves behind his mask for Speed to find. Knowing that the honorable and great racer was his brother, Speed is left somewhat in happiness.
In the Speed Racer major motion picture directed by the Wachowski Brothers, Racer X is portrayed by actor Matthew Fox. In the film Rex Racer fakes his own death in a cross country race so that he can fight corruption and evil that plagues racing and the world.
He undergoes plastic surgery to alter his appearance so that there is no chance people can find out who he really is when he does not have his mask on. Speed figures out Racer X is his brother but when Racer X takes off his mask, Speed is fooled into believing he is wrong. Racer X never reveals his true identity to his family.